1. Field of the Invention
The present invention relates to a reference voltage generating circuit having a startup circuit.
2. Description of the Related Art
FIG. 2 shows a configuration example of a reference voltage generating circuit 200 and a startup circuit 210 thereof according to a prior art.
First, the configuration of the reference voltage generating circuit 200 will be described. A resistor 111 is connected between a gate of a p-channel MOS field-effect transistor (hereinafter, referred to as FET) 123 and an output terminal 140. A resistor 112 is connected between the gate of the MOSFET 123 and an anode of a diode 113. A cathode of the diode 113 is connected to a ground potential. A resistor 114 is connected between a gate of a p-channel MOSFET 124 and the output terminal 140. A diode 115 has an anode connected to the gate of the MOSFET 124 and a cathode connected to the ground potential.
A differential amplifier 120 has a direct-current power supply 121 and MOSFETs 122 to 126. The direct-current power supply 121 has an anode connected to a gate of the p-channel MOSFET 122 and a cathode connected to the ground potential. The p-channel MOSFET 122 has a source connected to a power supply potential and a drain connected to a junction point between sources of the p-channel MOSFETs 123, 124. Gates of the n-channel MOSFETs 125, 126 are connected to each other, and a junction point therebetween is connected to a drain of the MOSFET 123. The n-channel MOSFET 125 has a drain connected to the drain of the MOSFET 123 and a source connected to the ground potential. The n-channel MOSFET 126 has a drain connected to a drain of the MOSFET 124 and a source connected to the ground potential.
An output amplifier 130 has MOSFETs 131, 132. The p-channel MOSFET 131 has a gate connected to the anode of the direct-current power supply 121, a source connected to a power supply potential, and a drain connected to the output terminal 140. The n-channel MOSFET 132 has a gate connected to the drain of the MOSFET 124, a drain connected to the output terminal 140, and a source connected to the ground potential.
Next, the configuration of the startup circuit 210 will be described. A direct-current power supply 211 has an anode connected to a gate of a p-channel MOSFET 212 and a cathode connected to the ground potential. The p-channel MOSFET 212 has a source connected to a power supply potential and a drain connected to a drain of an n-channel MOSFET 213. The n-channel MOSFET 213 has a gate connected to the output terminal 140 and a source connected to the ground potential. Gates of p-channel MOSFETs 214, 216 are connected to each other and a junction point therebetween is connected to a drain of the MOSFET 214. The MOSFET 214 has a source connected to the power supply potential and a drain connected to a drain of an n-channel MOSFET 215. The MOSFET 215 has a gate connected to the drain of the MOSFET 212 and a source connected to the ground potential. A MOSFET 216 has a source connected to the power supply potential and a drain connected to a gate of the MOSFET 124.
FIG. 3A shows output voltages 302a, 302b of the output terminal 140 of the reference voltage generating circuit 200 when the startup circuit 210 is not provided. The horizontal axis represents time [t] after power application, and the vertical axis represents voltage [V]. After the power application, a power supply voltage 301 gradually increases and is stabilized at Vcc [V] before long. The reference voltage generating circuit 200 outputs one of two kinds of the output voltages 302a, 302b according to manufacturing variation or the like. It is not possible to assure which one of two kinds of the output voltages 302a, 302b is outputted. As a result, the output voltage (reference voltage) of the reference voltage generating circuit 200 has two stabilization points 0 [V] and Vo [V]. Vo [V] is a desired stabilization point of the output voltage and 0 [V] is an undesired stabilization point of the output voltage.
FIG. 3B, which corresponds to FIG. 3A, shows the output voltage 302a of the output terminal 140 when the startup circuit 210 is connected to the reference voltage generating circuit 200. The startup circuit 210 is capable of leading the output voltage of the reference voltage generating circuit 200 to the one desired stabilization point Vo [V] out of the two stabilization points 0 [V] and Vo [V]. This can ensure that the reference voltage generating circuit 200 outputs the desired output voltage 302a in accordance with the increase in the power supply voltage 301.
Next, the operation of the reference voltage generating circuit 200 without the startup circuit 210 will be described. The reference voltage generating circuit 200 has a two-stage amplification circuit constituted of the differential amplifier 120 and the output amplifier 130. When an offset does not exist in the differential amplifier 120, the output voltage 302a is outputted, whereas, when the offset exists, the output voltage 302b is outputted. Specifically, in the differential amplifier 120, the MOSFETs 125, 126 function as current mirrors, and a voltage according to the output voltage of the output terminal 140 is feedback-inputted to the gates of the differential input MOSFETs 123, 124 of the differential amplifier 120, so that the MOSFETs 123, 124 are feedback-controlled to have equal drain voltages. When the drain voltages of the differential input MOSFETs 123, 124 are the same, which means that no offset exists, the output voltage 302a is outputted. On the other hand, when an offset of difference in drain voltage between the MOSFETs 123, 124 by a predetermined voltage (for example, 10 mV) or more exists due to manufacturing variation or the like, the output voltage 302b is outputted.
When the output voltage 302a in FIG. 3A is outputted, the power supply voltage 301 gradually increases after the power application, so that a voltage of the direct-current power supply 121 also gradually increases. Then, the output voltage 302a of the output terminal 140 also gradually increases and before long, the output voltage 302a is kept at the reference voltage Vo.
On the other hand, when the output voltage 302b in FIG. 3A is outputted, the power supply voltage 301 gradually increases after the power application and the output voltage 302b of the output terminal 140 also increases. Here, an offset exists in the differential amplifier 120, resulting in difference in drain voltage between the MOSFETs 123, 124. For example, the drain voltage of the MOSFET 123 becomes a low voltage as a minus offset, while the drain voltage of the MOSFET 124 becomes a high voltage as a plus offset. Then, the n-channel MOSFET 132 operates so as to turn on, so that the output voltage 302b of the output terminal 140 decreases. The decrease in the output voltage 302b causes by feedback the decrease in gate voltages of the input MOSFETs 123, 124 of the differential amplifier 120, and then the drain voltage of the p-channel MOSFET 124 increases. By this feedback loop, the output voltage 302b gradually decreases to stabilize at 0 [V] before long.
Next, the operation of the reference voltage generating circuit 200 with the startup circuit 210 connected thereto will be described. In this case, the output voltage 302a in FIG. 3B is outputted. After the power application, the power supply voltage 301 and the output voltage 302a gradually increase. The voltage 302a of the output terminal 140 of the reference voltage generating circuit 200 is monitored at the gate of the n-channel MOSFET 213 that is an input of the startup circuit 210. When the output voltage 302a of the reference voltage generating circuit 200 is lower than a threshold voltage of the n-channel MOSFET 213, the MOSFET 213 turns off. At this time, the gate voltage of the n-channel MOSFET 215 increases since the p-channel MOSFET 212 is a constant-current source, so that the MOSFET 215 turns on and a current flows therethrough. This current also flows through the p-channel MOSFET 214. Then, the same current also flows through the p-channel MOSFET 216 that has a current-mirror relation with the p-channel MOSFET 214. The current flowing through the p-channel MOSFET 216 increases the gate voltage of the input MOSFET 124 of the differential amplifier in the reference voltage generating circuit 200. As a result, a gate voltage of the n-channel MOSFET 132 lowers to increase the output voltage 302a of the reference voltage generating circuit 200.
When the output voltage 302a of the reference voltage generating circuit 200 increases to be equal to or more than the threshold voltage of the n-channel MOSFET 213 in the startup circuit 210, the n-channel MOSFET 213 turns on and the n-channel MOSFET 215 turns off due to the decrease in its gate voltage. Accordingly, the p-channel MOSFET 216 also turns off, so that the startup circuit 210 is disconnected from the reference voltage generating circuit 200 and the startup operation is stopped. Thereafter, by the operation of the reference voltage generating circuit 200, the output voltage 302a reaches the stabilization point Vo [V]. As described above, the startup circuit 210 operates so as to increase the output voltage 302a when the output voltage 302a of the reference voltage generating circuit 200 is low, and when the output voltage 302a becomes sufficiently high, it operates so as to stop the startup operation.
The following patent document 1 also discloses in FIG. 5 a startup circuit of a reference voltage generating circuit.
(Patent Document 1) Japanese Patent Application Laid-open No. 2000-181554